Photoluminescent phosphors

ABSTRACT

An X-ray imaging system includes a phosphor that exhibits a fast-acting photoluminescent response both in luminescing upon X-ray stimulation and ceasing to luminesce upon cessation of X-ray stimulation. The phosphor has the general formula A 2  MX 6  wherein A is selected from Cs, Rb, Na and K; M is selected from Ti, Zr, Hf and Te; and X is selected from Cl and Br. In one form the phosphor has a purity with respect to naturally-occuring impurities of at least about 98.0 percent (by weight) and is sufficiently deficient of luminescent activators effective only at very low temperatures that the phosphor luminesces at higher temperatures.

This application is a continuation of application Ser. No. 303,973,filed Jan. 30, 1989, now abandoned, which is a continuation ofapplication Ser. No. 102,817, filed Sept. 22, 1987, now abandoned, whichis a continuation of application Ser. No. 818,856 filed Jan. 13, 1986,now abandoned, which is a continuation of application Ser. No. 653,981,filed Sept. 24, 1984, now abandoned, which is a division of applicationSer. No. 528,830, filed Sept. 2, 1983, now U.S. Pat. No. 4,496,844.

BACKGROUND OF THE INVENTION

The present invention relates to radiographic X-ray imaging systems and,more particularly, to photoluminescent phosphors especially suitable foruse in X-ray imaging systems at temperatures in excess of about 77°kelvin.

X-ray imaging systems are useful for producing images of, for example,internal organs of a human body. The basic principle of their operationinvolves the passing of X-rays through an object of inquiry (e.g., ahuman body), which X-rays then impinge on a photoluminescent layer. Theinternal parts of the object of inquiry absorb some of the X-rays as afunction of their structure, whereby the X-ray pattern impinging on thephotoluminescent layer is representative of the structure of suchinternal parts.

The X-ray pattern on the photoluminescent layer stimulates a luminescent(or light) image in the photoluminescent layer. The material of thephotoluminescent layer that luminesces is known as a phosphor. It isdesirable that the phosphor exhibit a "fast-acting" response tostimulation by X-rays; that is, as used herein, a rapid response both inluminescing upon excitation by X-rays and ceasing to luminesce uponcessation of X-ray excitation. This is because typical X-ray imagingsystems incorporate sophisticated and expensive digital computerapparatus that are most economically operated at high speed.

Typical known phosphors that are composed of ceramic exhibit arelatively slow-acting response to X-ray stimulation. This increases theexpense of operation of a typical imaging system including complexdigital computer apparatus that is costly to operate. It would thus bedesirable to provide a phosphor that exhibits a fast-acting response toX-ray stimulation.

A further class of known phosphors are effective only at temperaturesbelow about 77° kelvin and are doped with luminescent activators. Thesephosphors comprise, for example: Cs₂ ZrCl₆ doped with one of Ir, Os, Re.Pt, Mo, Ru or U; Cs₂ HfCl₆ doped with Os, Mo or Re; and Cs₂ SnCl₆ dopedwith Ir, U, Re or Os. These doped phosphors are known to exhibitluminescence at about 4.4° kelvin (liquid helium temperature), but notat about 77° kelvin (liquid nitrogen temperature) or higher temperaturesdue to a parasitic effect within the host material. Accordingly, theyare not suitable for use in typical X-ray imaging systems, at leastwithout employing expensive cooling apparatus to maintain the requiredcold temperature.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the present invention is to provide an X-ray imaging systemutilizing a phosphor with a fast-acting response to X-ray stimulation.

A further object of the invention is to provide an X-ray imaging systemutilizing an undoped phosphor that luminesces at temperatures aboveabout 77° kelvin.

The foregoing objects are achieved in an X-ray imaging system, which, ina preferred form, comprises a photoluminescent layer including aphosphor of the general formula:

    A.sub.2 MX.sub.6

wherein A is cesium (Cs), Rubidium (Rb), potassium (K) or sodium (Na);

M is titanium (Ti), zirconium (Zr), hafnium (Hf) or tellurium (Te); and

X is chlorine (Cl) or bromine (Br).

The phosphor preferably has a purity with respect to naturally-occurringimpurities of at least about 98.0 percent by weight and is sufficientlydeficient of luminescent activators effective at temperatures belowabout 77° kelvin that the phosphor exhibits luminescence at temperaturesgreater than about 77° kelvin.

An X-ray source is included and is adapted to expose thephotoluminescent (PL) layer to X-rays passing through an object andthereby to stimulate emission of a light image from the PL layer. Thelight image is then read by a photodetection means, such as a photodiodedevice.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims setting forth theparticular features of the invention which are deemed to be novel, it isbelieved that the organization and method of using the invention will bebest understood from considering the following description in connectionwith the drawings, in which:

FIG. 1 is a schematic illustration partially in block diagram form of aradiographic imaging system in accordance with the present invention;

FIG. 2 is a cross-sectional view illustrating an X-ray screenconstruction utilizing the phosphor materials of the present invention;

FIG. 3 is a graph of luminescent output versus purity of a particularphosphor of the present invention;

FIG. 4 is a graphical illustration of the luminescent spectral output ofa phosphor of the present invention as well as a phosphor according tothe prior art that includes a luminescent activator;

FIG. 5 is a schematic depiction of a zone refining apparatus useful inpurification of the phosphor of the present invention; and

FIG. 6 is an illustration of luminescent output intensity of a phosphorof the present invention in response to X-ray stimulation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, FIG. 1 schematically illustrates aradiographic imaging system 10 suitable for real time use and whichutilizes a photoluminescent layer (PL) 12 incorporating one or more ofthe phosphors of the present invention, which are described in detailbelow. PL layer 12 is held in position by a physical support 14 that istransparent to X-rays, and is adapted to receive X-rays 16 from an X-raysource 18. X-rays 16 are transmitted through an object of investigation20, such as a human body. PL layer 12 is stimulated intophotoluminescence by X-rays 16 that have passed through object 12 atdifferent levels and exhibits a light image representative of theinternal parts of object 20. A photodetection means 22 is providedadjacent PL layer 12 so as to convert the light image information on PLlayer 12 to a digitized electrical image, which is then transmitted todigital processor 24 in order to enhance the "quality" (i.e. clarity) ofthe digitized electrical image. The resulting digitized electrical imagefrom digital processor 24 is then transmitted to a digital imager 26 inorder to provide a final digitized electrical image correcting certaindeficiencies found in the original light image exhibited on PL layer 12.

Photodetection means 22 may comprise, by way of example, a photodiodedevice, a photomultiplier device, or a charge-coupled device, each ofwhich are known in the art. Digital processor 24 and digital imager 26each may suitably comprise conventional devices.

PL layer 12 preferably comprises a monocrystalline layer of one or moreof the inventive phosphors that are described in detail below.Alternatively, PL layer 12 may comprise a polycrystalline layer of oneor more of the inventive phosphors in particle form that are boundwithin an optically isotropic plastic layer, such as a polyvinylnapthalene toluene copolymer. A further alternative embodiment of PLlayer 12 comprises one or more of the present inventive phosphors inpolycrystalline, particle form, suitably adhered to support 14 of Mylarplastic film, for example, by an adhesive such aspolymethylmethacrylate.

In FIG. 2 there is depicted a self-supporting representative X-rayscreen construction according to the present invention, that is usefulto permanently record a photoluminescent light image on photographicfilm. Specifically, the X-ray screen construction of FIG. 2 utilizes apair of flexible backing layers 30 and 32 along with a pair of opticalreflecting layers 34 and 36 and a further pair of photoluminescentlayers 38 and 40, as illustrated, to expose an intermediate,double-emulsion photographic film member 42. As used herein,"photodetection means" is intended to embrace such photographic filmmember 42.

In accordance with the present invention, the photoluminescent layersused in the above-described radiographic imaging system (FIG. 1) andX-ray screen construction (FIG. 2) include at least one phosphor of thegeneral formula:

    A.sub.2 MX.sub.6

wherein, A is selected from the group consisting of cesium, rubidium,potassium, or sodium; M is selected from the group consisting oftitanium, zirconium, hafnium or tellurium; and X is selected from thegroup consisting of chlorine or bromine.

In a more preferred embodiment of the invention A can be cesium,rubidium, or potassium; M can be zirconium, hafnium, or tellurium; and Xis again selected from the group consisting of chlorine and bromine.

In a still more preferred embodiment of the invention, the phosphorcomposition is one in which A is cesium or rubidium; M is zirconium,hafnium, or tellurium; and X is chlorine or bromine.

In a particularly preferred aspect of the invention the phosphorcomposition of the photoluminescent layer can be characterized by thegeneral formula:

    A.sub.2 M'.sub.1-x M".sub.x X.sub.6

wherein A, M and X are defined above. Illustrative compositions include:

Cs₂ Hf_(1-x) Te_(x) Cl₆ and

Cs₂ Zr_(1-x) Te_(x) Cl₆ where it is especially preferred that

005≦x≦0.1

Phosphors of the foregoing formulas constitute stoichiometricallydifferent materials that are related in that they crystallize with thecubic K₂ PtCl₆ type structure. Each phosphor preferably has a puritywith respect to naturally-occurring impurities of at least about 98.0percent by weight. By "naturally-occurring impurities" is meantimpurities typically found in the reagents used to produce a phosphor ofthe present invention. The importance of attaining the foregoing, highdegree of purity in the present phosphor (or phosphors) is made apparentfrom the graph of FIG. 3, illustrating the luminescent output of aphosphor of the present invention for different degrees ofnaturally-occurring impurities (by weight).

From FIG. 3 it can be appreciated that a phosphor of the presentinvention exhibits a luminescent output of about 5 percent of themaximum possible when its purity with respect to naturally-occurringimpurities is about 98.0 percent. The higher the purity of the phosphor,the higher its luminescent output. It is particularly preferred that thepurity of the foregoing phosphor be about 99.99 percent, whichadvantageously results in the attainment of about 99.9 percent of themaximum luminescent output. The data of FIG. 3 has been verified for thefollowing phosphors: Cs₂ ZrCl₆, Cs₂ HfCl₆ and Cs₂ Zr.sub..99 Te.sub..01Cl₆. It is thus reasonable to expect similar performance from the otherphosphors of the foregoing formula.

In order for the phosphors of the present invention to luminesce attemperatures above about 77° kelvin, the phosphors must additionally besufficiently deficient, or free of, the luminescent activators discussedabove with respect to one class of prior art phosphors. Such luminescentactivators do not occur naturally in the reagents used to produce thepresent phosphors. As noted above, these prior art luminescentactivators are effective at about 4.4° kelvin (liquid heliumtemperature) but not at about 77° kelvin (liquid nitrogen temperature).A tolerable impurity level of a luminescent activator is one that issufficiently low whereby the phosphor exhibits luminescence attemperatures of about 77° kelvin or above. The maximum tolerableimpurity level of a prior art luminescent activator will be apparent tothose skilled in the art based upon (1) the above-discussed, distincttemperature range wherein a sufficiently pure phosphor luminesces (i.e.,at temperatures greater than about 77° kelvin) and (2) the uniqueluminescent spectrum exhibited by a sufficiently pure sample of thepresent phosphor Cs₂ HfCl₆, by way of example. This particular phosphor,when sufficiently deficient of luminescent activators effective only attemperatures below about 77° kelvin (and sufficiently pure with respectto naturally-occurring impurities), exhibits the luminescent spectrum50, shown in FIG. 4, whereas a phosphor with the same formula (i.e. Cs₂HfCl₆), when doped with the prior art luminescent activator Os exhibitsthe distinctly different luminescent spectra 52, shown in FIG. 4. Thespectrum 50 comprises a single continuous broad emission, while thespectra 52 comprises a multiplicity of individual narrow band emissions.

A preferred method of preparing the present phosphor of the generalformula A₂ MX₆, as discussed in detail above, comprises thoroughlymixing two moles of alkali metal halide (AX) such as CsCl with one moleof a non-alkali metal halide (MX₄) such as HfCl₄ in a sealed evacuatedsilica tube, and then raising this mixture at least momentarily to about850° C., preferably at a rate limited to about 25° C. per hour. Thisprocedure produces one mole of Cs₂ HfCl₆. The mixture of alkali metalhalide and non-alkali metal halide can be raised in temperature at ratesother than the preferred rate of 25° C. per hour; a faster rate resultsin a more volatile, possibly explosive, reaction between the alkalimetal halide and the non-alkali metal halide, and a slower rate requiresmore time to produce the phosphor. Lesser maximum temperatures can beused, although the mixture would need to be maintained at the lessermaximum temperature for a longer time to permit the phosphor to beproduced. For example, the alkali metal halide and non-alkali metalhalide mixture could be held at about 805° C. for a period of about 24hours to produce the desired phosphor.

The foregoing processing temperatures are considerably lower than thoserequired to produce prior art ceramic phosphors and, as such, thephosphors of the present invention are more economical to fabricate thatceramic phosphors. The present invention comprises the mixing of onemole of alkali metal halide with two moles of non-alkali metal halide inan aqueous solution, with the desired phosphor precipitating from theaqueous solution. This procedure, however, typically results in aphosphor having a higher impurity level than a phosphor producedaccording to the foregoing procedure involving heating of the alkalimetal halide and non-alkali metal halide mixture.

Phosphors made by the foregoing phosphor-producing procedures arepolycrystalline in form and typically contain naturally-occurringimpurities. A preferred technique for purifying the resultingpolycrystalline mass is the technique of zone refining, which may becarried out in the apparatus schematically illustrated in FIG. 5.According to the technique of zone refining, a polycrystalline mass ofphosphor 60 is placed within a silica (or other suitable refractorymaterial) tube 62. A heater 64 surrounds tube 62 and is effective totransform the portion of polycrystalline mass 60 in the vicinity ofheater 64 into molten metal 66. Impurities from polycrystalline mass 60then diffuse into molten metal 66. Heater 64 is moved in the directionof arrow 68 from one end of tube 62 to the other. Further heaters, suchas heater 70, can be employed so as to reduce the overall purificationtime, with six heaters being preferred. Alternatively, three heaterscould be passed from one end of tube 62 to the other, by way of example.

By using the foregoing technique of zone refining, phosphors with apurity of about 99.99 percent (by weight) with respect tonaturally-occurring impurities have been obtained. Further details ofthe technique of zone refining are contained in Pfann Zone Melting, 2ndEd., New York: Wiley Interscience (1965), which is incorporated hereinby reference.

After purification, the polycrystalline mass of phosphor can be used inits existing particle form. Alternatively, the pholycrystalline mass canbe formed into a single crystal in accordance with the Bridgmancrystallization technique, by way of example. This technique (notillustrated herein) involves placement of the polycrystalline mass intoa specially shaped tube and then inserting the tube into a furnace at atemperature preferably in the range from about 810° C. to about 830° C.The Bridgman technique is described more fully in W. O. Lawson and S.Nielsen, Preparation of Single Crystals, London: Butterworths ScientificPublications (1958) (see especially discussion commencing on page 14),which is incorporated herein by reference. After crystallization, theresulting crystal is cut into a desired shape for use in a radiographicimaging system such as described above.

Phosphors of the present invention have been found to exhibit highlydesirable characteristics for use in radiographic imaging systems, asexplained presently with reference to FIG. 6. Intervals of X-rayemission from an X-ray source (not shown) are depicted in FIG. 6 alongwith the luminescent output of a phosphor that is stimulated intophotoluminescence by these X-rays. Important characteristics of theluminescent output are denoted on the luminescent output waveform anddiscussed below.

Of particular importance in determining the speed at which X-ray imagingmay occur is the "primary decay" of the luminescent output, which is thetime it takes for the luminescent output to decay to 1/e (about 37percent) of its maximum value after cessation of X-ray stimulation. Ofconsiderable importance also is the "afterglow" of the phosphor, whichis a spurious or unwanted continuation of luminescence of the phosphorafter a relatively long interval after cessation of an X-ray stimulationsignal. Of importance in resolution quality of a radiographic image isthe "hysteresis", or change in maximum luminescent output, betweensuccessive periods of photoluminescence of a phosphor. The foregoingcharacteristics of the present phosphors are tabulated as follows forvarious, preferred phosphors of the present invention, along with thecharacteristic of phosphor efficiency in terms of percent of theindustrial standard phosphor CsI:Tl.

    ______________________________________                                               Primary    Afterglow                                                   Phosphor                                                                             Decay      @ 10 msec.                                                                              Hysteresis                                                                             Efficiency                               ______________________________________                                        Cs.sub.2 HfCl.sub.6                                                                  <0.1 msec. 1%        <1%      40%                                      Cs.sub.2 ZrCl.sub.6                                                                  <0.1 msec. 0.5%      <1%      40-55%                                   Cs.sub.2 HfBr.sub.6                                                                  <0.1 msec. 1%         2%      45%                                      ______________________________________                                    

EXAMPLE 1: Preparation of Cs₂ ZrCl₆ in Single Crystal Form

A single crystal of 11.38 g Cs₂ ZrCl₆ was prepared by first placing4.669 g of ZrCl₄ of 99.99 percent purity into a clear, dry 1/2-inchdiameter fused silica tube with 6.72 g of CsCl of 99.9999 purity. Thepurity levels in these examples are in terms of weight and concernnaturally-occurring impurities. The tube and contents were evacuated to10μ Hg, at which point the tube was sealed off from the vacuum line atan overall tube length of 8 inches. The tube was placed in a tubularresistance furnace equipped with explosion shields. The temperature wasraised at 25° C. per hour to 830° C. and maintained at this temperaturefor two hours. Then the furnace was cooled at 100° C. per hour to roomtemperature (21° C.) and the tube removed from the furnace. The tube wasopened carefully to allow any HCl gas present to safely escape and thenthe polycrystalline Cs₂ ZrCl₆ was mechanically extracted.

This polycrystalline mass of Cs₂ ZrCl₆ was ground in an agate mortar andthen placed in a clean, dry 1/2-inch diameter silica tube forpurification in accordance with the zone refining technique. The tubewas evacuated to 1μ Hg then sealed off from the vacuum source. The tubewas raised at 1/2-inch per hour through two heated zones each being9/16-inch in diameter x 1/2-inch length and spaced 3 inches from eachother. The temperature at the center of the zones was 830° C. Thispurification procedure was repeated for two more passes. The tube wasagain carefully opened and the purified polycrystalline Cs₂ ZrCl₆extracted.

For crystal growth, the pure Cs₂ ZrCl₆ was placed in a clean, drytapered silica tube (Bridgeman type) and evacuated to 1 Hg. The tube wassealed and then lowered at 1/4 inch per hour through a furnace at 830°C., the furnace having a uniform temperature zone at least 10 incheslength by 1-inch diameter. After this heating step, a single crystal ofCs₂ ZrCl₆ was mechanically extracted after opening the tube.

EXAMPLE 2: Preparation of Cs₂ HfCl₆ in Single Crystal Form

A single crystal of 13.12 g Cs₂ HfCl₆ was prepared in accordance withthe procedure of Example 1, above, as modified by using 6.40 g of HfCl₄of 99.99 percent purity in place of the 4.66 g of ZrCl₄.

EXAMPLE 3: Preparation of Cs₂ HfBr₆ in Particle Form

The phosphor Cs₂ HfBr₆ in particle form was prepared by first mixing4.98g of HfBr₄ of 99.999 percent purity with 4.26g of CsBr and placingthe resulting mixture into a clean, dry silica tube 1 inch in length.The tube was evacuated to 10μ Hg and then sealed. The tube and itscontents were heated to 830° C. at 25° C. per hour in a furnace equippedwith explosion shields. After maintaining the tube and contents at 830°C. for 1 hour, the tube was cooled at 100° C. per hour to roomtemperature (21° C.). The tube was then carefully opened and thepolycrystalline Cs₂ HfBr₆ extracted.

EXAMPLE 4: Preparation of Cs₂ TeCl₆ in Particle Form

The phosphor Cs₂ TeCl₆ in particle form was prepared by first mixing6.72 g of CsCl and 5.38 g of TeCl₄ and placing the resulting mixtureinto a clean, dry silica tube, which was then evacuated, heated andcooled as in Example 3, above.

EXAMPLE 5: Preparation of Cs₂ Zr.sub..99 Te.sub..01 Cl₆ in Particle Form

The phosphor Cs₂ Zr.sub..99 Te.sub..01 Cl₆ in particle form was preparedby first mixing 6.72 g of CsCl, .054 g TeCl₄ and 4.61 g ZrCl₄ andplacing the resulting mixture into a clean, dry silica tube, which wasthen evacuated, heated and cooled as in Example 3, above.

EXAMPLE 6: Preparation of Cs₂ Hf.sub..99 Te₀.1 Cl₆ in Particle Form

The phosphor Cs₂ Hf.sub..99 Te.sub..01 Cl₆ in particle form was preparedby first mixing 6.72 g of CsCl, .054 gf TeCl₄ and 6.34 g HfCl₄ andplacing the resulting mixture into a clean, dry silica tube, which wasthen evacuated, heated and cooled as in Example 3, above.

In each of the preceding examples, all steps were preformed in a dry N₂atmosphere.

The foregoing describes phosphors that are particularly suitable for usein radiographic imaging systems because they can be fabricated moreeconomically than present ceramic phosphors due to lower fabricationtemperatures and are fast-acting, thereby permitting increased operatingspeed of a radiographic imaging system in which a phosphor (orphosphors) is utilized.

While the invention has been described with respect to specificembodiments, many modifications and substitutions of the invention willbe apparent to those skilled in the art. It is, therefore, to beunderstood that the appended claims are intended to cover all suchmodifications and substitutions as fall within the true spirit and scopeof the invention.

What is claimed is:
 1. A photoluminescent phosphor having the formula:

    Cs.sub.2 Hf.sub.1-x Te.sub.x Cl.sub.6 or Cs.sub.2 Zr.sub.1-x Te.sub.x Cl.sub.6

wherein 0.005≦X≦0.1, said phosphor having a purity with respect tonaturally-occurring impurities of at least about 98.0% by weight andbeing sufficiently deficient of luminescent activators effective only attemperatures below about 77° Kelvin that said phosphor exhibitsluminescence in response to X-ray bombardment at temperatures greaterthan about 77° Kelvin.
 2. The photoluminescent phosphor of claim 1having the formula:

    Cs.sub.2 Hf.sub.1-x Te.sub.x Cl.sub.6

wherein 0.005≦X≦0.1.
 3. The photoluminescent phosphor of claim 1 havingthe formula:

    Cs.sub.2 Zr.sub.1-x Te.sub.x Cl.sub.6

wherein 0.005≦X≦0.1.
 4. The photoluminescent phosphor of claim 1 whereinsaid phosphor has a purity with respect to naturally-occurringimpurities of at least about 99.5%.
 5. The photoluminescent phosphor ofclaim 1 wherein said phosphor exhibits a luminescent spectrum comprisinga single, continuous, broad emission.